In this paper, an in-band and out-of-band microwave wireless power-transmission characteristic analysis of a slot ring radome based on an approximate analytical method is proposed. The main contribution of this paper is that, in the approximate analysis of the ring radome, a unified expression of the incident field on the radome surface is derived with E-plane and H-plane scanning, and the ring is approximated as 30 segments of straight strips. Solving the corresponding 60 ×
60 linear equations yields the electric current distribution along the ring strip. The magnetic current along the complementary slot ring is obtained by duality. Thanks to the fully analytical format of the current distribution, the microwave wireless power-transmission characteristics are efficiently calculated using Munk's scheme. An example of a slot ring biplanar symmetric hybrid radome is used to verify the accuracy and efficiency of the proposed scheme. The central processing unit (CPU) time is about 690 s using Ansys HFSS software versus 2.82 s for the proposed method.

This work presents an optimal design method of antenna aperture illumination for microwave power transmission with an annular collection area. The objective is to maximize the ratio of the power radiated on the annular collection area to the total transmitted power. By formulating the aperture amplitude distribution through a summation of a special set of series, the optimal design problem can be reduced to
finding the maximum ratio of two real quadratic forms. Based on the theory of matrices, the solution to the formulated optimization problem is to determine the largest characteristic value and its associated characteristic vector. To meet security requirements, the peak radiation levels outside the receiving area are considered to be extra constraints. A hybrid grey wolf optimizer and Nelder–Mead simplex method is developed to deal with this constrained optimization problem. In order to demonstrate the effectiveness of the proposed method, numerical experiments on continuous apertures are conducted; then, discrete arrays of isotropic elements are employed to validate the correctness of the optimized results. Finally, patch arrays are adopted to further verify the validity of the proposed method.

An experimental study is conducted on several retro-reflective beamforming schemes for wireless power transmission to multiple wireless power receivers (referred to herein as "targets"). The experimental results demonstrate that, when multiple targets broadcast continuous-wave pilot signals at respective frequencies, a retro-reflective wireless power transmitter is capable of generating multiple wireless power beams aiming at the respective targets as long as the multiple pilot signals are explicitly separated from one another by the wireless power transmitter. However, various practical complications are identified when the pilot signals of multiple targets are not appropriately differentiated from each other by the wireless power transmitter. Specifically, when multiple pilot signals are considered to be carried by the same frequency, the wireless power transmission performance becomes heavily dependent on the interaction among the pilot signals, which is highly undesirable in practice. In conclusion, it is essential for a retro-reflective wireless power transmitter to explicitly discriminate multiple targets' pilot signals among each other.

Ultrashort pulse transmission has been recognized as a primary problem that fundamentally hinders the development of ultrafast electronics beyond the current nanosecond timescale. This requires a transmission line or waveguide that exhibits an all-pass frequency behavior for the transmitted ultrashort pulse signals. However, this type of waveguiding structure has not yet been practically developed; groundbreaking innovations and advances in signal transmission technology are urgently required to address this scenario. Herein, we present a synthesized all-pass waveguide that demonstrates record guided-wave controlling capabilities, including eigenmode reshaping, polarization rotation, loss reduction, and dispersion improvement. We experimentally developed two waveguides for use in ultrabroad frequency ranges (direct current (DC)-to-millimeter-wave and DC-to-terahertz). Our results suggest that the waveguides can efficiently transmit picosecond electrical pulses while maintaining signal integrity. This waveguide technology is an important breakthrough in the evolution of ultrafast electronics, providing a path towards frequency-engineered ultrashort pulses for low-loss and low-dispersion transmissions.

While sufficient review articles exist on inductive short-range wireless power transfer (WPT), long-haul microwave WPT (MWPT) for solar power satellites, and ambient microwave wireless energy harvesting (MWEH) in urban areas, few studies focus on the fundamental modeling and related design automation of receiver systems. This article reviews the development of MWPT and MWEH receivers, with a focus on rectenna design automation. A novel rectifier model capable of accurately modeling the rectification process under both high and low input power is presented. The model reveals the theoretical boundary of radio frequency-to-direct current (dc) power conversion efficiency and, most importantly, enables an automated system design. The automated rectenna design flow is sequential, with the minimal engagement of iterative optimization. It covers the design automation of every module (i.e., rectifiers, matching circuits,   antennae, and dc–dc converters). Scaling-up of the technique to large rectenna arrays is also possible, where the challenges in array partitioning and power combining are briefly discussed. In addition, several cutting-edge rectenna techniques for MWPT and MWEH are reviewed, including the dynamic range extension technique, the harmonics-based retro-directive technique, and the simultaneous wireless information and power transfer technique, which can be good complements to the presented automated design methodology.